I got BBC in my a levels, and contacted the vet schools that I had applied to to see if they would reconsider me (edinburgh, glasgow, bristol and liverpool.) All of them said no - that they wouldn't look at retakes in all three a levels, and that there were 'enough people who got high enough grades to start with, without having to look at other people.' I had been ill from my last year of school with a back problem - which later turned out to be a spinal bone tumour. I had managed AAB until the last set of exams in June of my A2 year, but which point I was in so much pain, and so drugged up that I really messed them up. I had an operation soon after, which sorted it all out, and I'm fine now, but even with letters from my doctor, consultant, etc they wouldn't consider me.
I don't mean to try and discourage you, but I would strongly advise that you make sure you contact the unis explain the situation and are sure that they would def consider you (make sure you explain about your GCSE results as well), other wise you could risk spending a lot of time and effort redoing them for them to say no at the end of it.